Electronics > Projects, Designs, and Technical Stuff
OPA2388: peak-hold detector circuit affected by opamp input leakage current
Kleinstein:
The diagram is the one attached. The 2 nd OP is working with feedback. The feedback reduces the phase shift for much of the lower frequencies. even the about 90 deg. Phase shift near 10 MHz is still critical and chances are the circuit is still at the edge of oscillation. This is why it would really help if the buffer is faster than the 1 st. OP. One may be able to slow down the 1 st OP a little with some capacitance or RC between the inputs. I am still not sure how much this help with the nonlinear FB.
One could consider a solution with a single OP and a JFET as buffer amplifier - chances are the JFET as a source follower is quite fast compared to the usual OPs. DC errors from the FET are still compensated by the OP.
AE7OO:
Hi,
I did not see a posted speed(or I missed it), so I'll go ahead throw my goto sample and hold into the mix.
Years ago I picked up a couple of tubes of NS LMC660's at a hamfest. You'll have problems finding anything with a lower input bias current(2fA) or input offset current(1fA). There is an app note floating around detailing the problems trying to measure them. It's offset voltage of 1mV is not too bad. Common mode input range includes ground or below.
When combined with a decent CMOS switch on the input(one of the 4066's is what I use) and a PP (or one of my horded Sprauge(sp) mil-spec hermetic polystyrenes, also hamfest) cap, and using deadbug construction makes a very low leakage S&H.
I've used it up to about 200Khz.
I just use the diagram from the datasheet.
brumbarchris:
--- Quote ---using deadbug construction makes a very low leakage S&H
--- End quote ---
Actually, I really need a peak detector, not just a S&H; unless, that is, if there is a way of determining when the maximum in the waveform occurs, in order to open the switch off the S&H at that particular moment.
--- Quote ---One could consider a solution with a single OP and a JFET as buffer amplifier
--- End quote ---
I eventually got it to work...sort of; using an Analog Devices AD8656ARMZ, which is foot compatible to the OPA2388. So it does work, but the error is unacceptable high. So I conclued that it is probably better to go and use to different opamps, as you suggest above. As I determined it with my experience in this circuit so far, the characteristics required for these to opamps are:
- first opamp in the chain (the one charging the cap) must not necessarily be a CMOS /JFET input one, but it must have:
- low offset voltage
- extremely good slew rate
- high output current
- RRIO (my input signal range is some 0...4.5V)
- second opamp in the chain must definitely be a CMOS / JFET input one and it must have:
- low input bias current
- lowest possible phase shift; this is actually critical. Every bit of phase shift will result in a delay in the feedback loop which essentially delays the first opamp to "sample" the input signal and recharge the capacitor; actually, this is a big problem I have, because I am not sure how to infer the phase shift of an opamp at Gain=1 from the information available in the datasheet. Most datasheets provide a graph with the phase shift in open loop conditions.
- RRIO
Best regards,
Cristian
Kleinstein:
Low phase shift in closed loop operation as a buffer kind of means high speed. So the 2 nd OP should be at least as fast as the 1st one, ideally more like > 5 times faster.
How critical the leakage current is depends on the hold time needed. So how long until the result is read. Depending on the application there can even be an advantage of there is a defined bias current (for one application I used an LF356 together with an LM318 - with the BJT intentionally as the buffer, as the bias provides a defined drift in the right direction). For a short time leakage is not that bad.
If long time analog hold is really needed one can add a second, slower stage.
The AD8656 is not such a bad choice.
brumbarchris:
I am not sure why you indicate the second opamp should be faster relative to the first one. I suppose low phase shift is something generally referenced vs the input signal frequency, rise time, etc. I mean, the voltage at the input of the second opamp will have about the same change rate as the signal at the input of the first opamp, so in this case I would consider the second opamp to be as low as possible in phase shift, period. If the first opamp is faster, then even better! Not so?
Cristian
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